strange brake problem

Can anyone suggest a cure for my UK Audi 80, I am at my wits end.
When pressing the brake pedal, the pedal was occasionally going nearly to

the floor, when pumped twice it would be OK and the brake pedal pressure
would be fine for days, I replaced the master cylinder with a new one from
Europarts UK and everything has been OK for a week, now the original problem
has happened again, brake pedal will go to the floor and pumped twice it
will have a good brake pedal and the brake pedal pressure
would be OK, brake pedal pressure is good and when braking car does not veer
from a straight line,
can anyone diagnose this problem as to why
brake pedal pressure keeps dropping from time to time? can't see any leaks,
master cyl reservoir fluid level is not dropping
Ps my audi 80 1.9TDI avant has got ABS discs brakes all round, 1994 model.
Thanks

Yes the clutch is fed from the reservoir too, nothing wrong with the clutch
operation, could this fault be caused by a sticking brake calliper?, the
brakes are ok when the brake pedal pressure is fine, does not pull from side
to side, just a thought? I will jack up the car and check each wheel, it's
annoying you want to know that you have good brakes all the time:))
Nad

Hi, brake level fluid never drops in reservoir, no leaks at any calliper
(pistons), today drove the car for 2 hours brake pedal firm, not spongy, car
brakes very well, after 2 hours, brake pedal goes to the floor little
pressure, pumped the brake pedal 2 twice got a firm brake pedal, then brake
pedal went to the floor again, pumped brake pedal twice got a firm brake
pedal again, brake pedal after driving another hour is firm and have good
brakes no sign of pressure dropping!!
Still looking for advice guys, I changed the master cylinder with a new one
when this symptom first happened, I guess the original problem was not the
master cylinder after all, I would have thought that problem is not a master
cylinder, since the problem still occurs.
checked so far..
No fluid leaks on ground.
Brake reservoir level does not drop.
car brakes in a straight line, tried this many times, perfect braking,
Handbrake (discs) holds car perfectly, and releases with no dragging, don't
think there is a seized piston (calliper)
What else is there to check,
brake pressure valve?
Servo? when the brake pedal pressure fell I stopped, switched off the engine
and pressed the pedal a couple of times and the pedal still went to the
floor
ABS???
Nad
<John> wrote in message

Suspect air in the system then.
Have you bled the system carefully, at master cylinder then each caliper in
turn? The order is Master Cyl / RH Rear / LH Rear / RH Front / LH Front.
When you are bleeding the rear cylinders you need to press the brake
regulator lever also. (you'll need an assistant!)

Could also be water in the (old?) brake fluid.
If cold no problem, after heating up it becomes steam which can be
pressed together ---> strange breaking behaviour.
(I hope you understand what I mean, english is not my native tongue)

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